Stratified NH and ND emission in the prestellar core 16293E in L1689N

Abstract

Context. High degrees of deuterium fractionation are commonly found in cold prestellar cores and in the envelopes around young protostars. As it brings strong constraints to chemical models, deuterium chemistry is often used to infer core history or molecule formation pathways. Whereas a large number of observations are available regarding interstellar deuterated stable molecules, relatively little is known about the deuteration of hydride radicals, as their fundamental rotational transitions are at high frequencies where the atmosphere is mostly opaque. Aims. Nitrogen hydride radicals are important species in nitrogen chemistry, as they are thought to be related to ammonia formation. Methods. We observed NH and ND in 16293E with the HIFI spectrometer on board the Herschel Space Observatory as part of the CHESS guaranteed time key programme, and derived the abundances of these two species using a non local thermodynamic equilibrium radiative transfer model. Results. Both NH and ND are detected in the source, with ND in emission and NH in absorption against the continuum that arises from the cold dust emission. Our model shows, however, that the ND emission and the NH absorption originate from different layers in the cloud, as further evidenced by their different velocities. In the central region of the core, we can set a lower limit to the [ND]/[NH] ratio of > ∼ 2%. This estimate is consistent with recent pure gas-phase models of nitrogen chemistry.

Modelling the molecular composition and nuclear-spin chemistryof collapsing pre-stellar sources★

“…Finally, in the case of the I16293E starless core, our model intercepts the observed ranges of column densities for o−NHD 2 and m−ND 3 at two densities, between approximately 10 6 and 10 7 cm −3 . Although the latter density may seem too high for this type of object, we note that it is close to the value of 1.4 × 10 7 cm −3 obtained from the dust-emission profiles (Bacmann et al 2016). Overall, the level of agreement with the observations reported in Fig.…”

Star Formation and Feedback: A Molecular Outflow–prestellar Core Interaction in L1689n

“…5 For a 16 K dust temperature (Stark et al 2004), we derive an H 2 column density of 1.8 × 10 23 cm −2 . However, if the dust temperature is 11 K, as suggested by Bacmann et al (2016), the corresponding column density increases to 3.3 × 10 23 cm −2 . Even in the latter case, the dust emission is optically thin (a 970 µm optical depth of ∼0.01).…”

“…However, these authors point out that a cold, high-density region with T d 16 K can easily be hidden at the center of the core. In fact, in a recent study, Bacmann et al (2016) derive a temperature profile increasing from 11 K at the core center to 16 K in the outer envelope. Even lower dust temperatures, in the range 8-16 K, are derived by Pagani et al (2016).…”

N2H+and N15NH+toward the prestellar core 16293E in L1689N

“…described in Bacmann et al (2016). We recall that the density is constant at ∼1.4 × 10 7 cm −3 within a radius of 4 and then A45, page 4 of 7 F. Daniel et al: N 2 H + in the prestellar core 16293E decreases outward.…”

“…We here analyzed the prestellar core of 16293E. To perform the analysis, we used the physical structure of 16293E described in Bacmann et al (2016). In this study, variations of the H 2 density and of the dust and gas temperature throughout the core were derived using continuum observations at wavelengths ranging from 160 µm to 1.3 mm.…”